The present invention generally relates to maximum likelihood detection methods and information recording and reproducing apparatuses, and more particularly to a maximum likelihood detection method which is suited for use in a partial response channel in which a trellis encoding is made and to an information recording and reproducing apparatus which employs such a maximum likelihood detection method.
In order to carry out high-density information recording and reproduction with respect to a magnetic disk, there is a known magnetic disk unit which employs the so-called partial response with maximum likelihood (PRML) technique which combines partial response and maximum likelihood detection. Recently, a trellis-coded partial response (TCPR) technique has been proposed which employs trellis encoding in the PRML technique so as to further improve the encoding gain.
In order to carry out the high-density information recording and reproduction with respect to the magnetic disk in the magnetic recording and reproducing apparatus, it is necessary to improve the data error rate in a state where the signal-to-noise (S/N) ratio is relatively poor. For this reason, it has recently become more and more important to record the information on the magnetic disk by adding an error correction code to the data. The TCPR technique combines to the partial response (1-D) channel a sliding block code which makes code values "0" and "1" correspond to the increase and decrease of the state, so that the hamming distance becomes larger than that in the case of the PRML technique and the error correction capability is improved.
The TCPR technique itself is proposed in Rae et al., "Design and Performance of a VLSI 120 Mb/s Trellis-Coded Partial Response Channel", IEEE Transactions on Magnetics, Vol.31, No.2, pp.1208-1214, March 1995, for example.
FIG. 1 is a system block diagram for explaining a conventional maximum likelihood detection method for a case where the TCPR technique is employed. In FIG. 1, a recording system of a magnetic disk unit includes a sliding block encoding circuit 501. On the other hand, a reproducing system of the magnetic disk unit includes a maximum likelihood detection circuit 511, a sliding block decoding circuit 512, and a code synchronizing circuit 513. The illustration of a magnetic disk and recording and reproducing heads is omitted in FIG. 1 for the sake of convenience. Instead, a path made up of the magnetic disk and the recording and reproducing heads is illustrated as a recording and reproducing (1-D) channel, that is, as a partial response (1-D) channel.
In FIG. 1, a numeral within brackets "{ }" indicates a state of the sliding block code, and it is assumed in this example for the sake of convenience that the sliding block code has 7 states {1} through {7}. As shown within the sliding block encoding circuit 501 and within the sliding block decoding circuit 512, when the state of the sliding block code is {4}, for example, the previous state is {5} if the previous code value is "0", and the previous state is {3} if the previous code value is "1". Similarly, when the state of the sliding block code is {4}, the next state is {3} if the present code value is "0" and the next state is {5} if the present code value is "1". However, when the state of the sliding block code is {1}, the previous code value can only be "0" and the previous state can only be {2}, and the present code value can only be "1" and the next state can only be {2}. In addition, when the state of the sliding block code is {7}, the previous code value can only be "1" and the previous state can only be {6}, and the present code value can only be "0" and the next state can only be {6}.
The state transition pattern of the sliding block code differs depending on whether the numeral indicating the state of the sliding block code is an odd number or an even number. For this reason, the maximum likelihood detection circuit 511 detects a maximum likelihood path from the state transitions by switching the state transition pattern depending on whether the numeral indicating the state of the sliding block code is an odd number or an even number. There are two kinds of state transition patterns P1 and P2, and these two state transition patterns P1 and P2 are alternately repeated. The switching of the state transition pattern is carried out based on a transition pattern switching signal received from the code synchronizing circuit 513. The sliding block decoding circuit 512 decodes a code string based on the code string from the maximum likelihood path detected by the maximum likelihood detection circuit 511 and a code conversion signal from the code synchronizing circuit 513.
The maximum likelihood detection circuit 511 detects the maximum likelihood path from the state transitions by switching the two kinds of state transition patterns P1 and P2 which are alternately repeated. In other words, the maximum likelihood detection circuit 511 detects the maximum likelihood path using the same number of states of trellis transitions as the number of states of the sliding block code. Consequently, it is necessary to synchronize the code string and the maximum likelihood detection circuit 511 before starting the detection of the maximum likelihood path. More particularly, it is necessary for the code synchronizing circuit 513 to detect a synchronizing word from the code string which is obtained from the recording and reproducing (1-D) channel, and to output the transition pattern switching signal to the maximum likelihood detection circuit 511 based on the detected synchronizing word.
Therefore, although the recording system adds the error correction code to the code string so that the error correction process can be made in the reproducing system, the maximum likelihood path cannot be detected in the maximum likelihood detection circuit 511 unless the synchronizing word of the code string is first reproduced. As a result, there was a problem in that the error correction process cannot be made with respect to the synchronizing word. In addition, since the conventional maximum likelihood detection method cannot carry out the error correction process with respect to the synchronizing word, there was a problem in that it is extremely difficult to further reduce the data error rate and to improve the reliability of the information recording and reproduction.